CRT devices have heretofore been used most widely as display devices of office appliances.
In the field of appliances, such as office appliances having display devices, there is an increasing demand in recent years for miniaturing or weight saving appliances or enlarging or thinning the screen of display devices. On that account, in place of conventional CRT devices, various new types of display devices have been developed with the view of answering to uses thereof or demands therefor. For instance, display devices such as liquid crystal display, plasma display, LED display, EL display and ECD display have been developed.
Of these display devices mentioned above, liquid crystal display basically has such a function that electric signals are given to switching elements comprising a liquid crystal compound, wherein the liquid crystal compound present in said switching elements is changed in its state, thereby to control the shielding and transmission of light and develop the electrical signals on the display device. Such liquid crystal display device as illustrated above has already been put to practical use not only as a display device of the aforementioned office appliances but also as a display device of digital watch or portable game and, at the same time, has come to be used recently as a display device of moving picture such as small-sized television.
In the above-mentioned display devices using liquid crystal compounds, various driving methods are already known. For example, as the driving method of liquid crystal display presently used, there is a TN (twisted nematic) mode. This TN mode is to carry out display by utilization of dielectric anisotropy of the molecule in the nematic phase of the liquid crystal compound, wherein the display is driven by energy proportional to the multiple of the electrical field given from outside (f.varies.E.sup.2).
In the switching elements of liquid crystal adopting this TN mode, however, there is involved such problems that in order to change the image being displayed, the driving time is prolonged, because the position of the molecule of the liquid crystal compound in the element must be changed, and also the voltage necessary for changing the position of the molecules of the liquid crystal compound, that is, the power consumption becomes large. In such switching elements as mentioned above, there is also such a problem that because the switching threshold value characteristic to the element is not so good, when the position of the molecules of the liquid crystal compound is changed at high speed and changeover operation is intended to carry out, leakage of voltage may occur and voltage may be given to the molecules which are not to be given the voltage and therefore contrast of the images may markedly decrease
Because of the above-mentioned problems associated with the conventional display method relying on TN mode as mentioned above, the display devices utilizing TN mode are not suitable particularly for large screen display devices or display devices for moving pictures such as small-sized digital television.
Furthermore, there are being used display devices utilizing STN (super twisted nematic) mode in which the switching threshold value and the like as mentioned above have been improved. By virture of utilization of such STN mode, a contrast of the display devices certainly improves because of its improved threshold value characteristics.
However, this display method is identical with the method utilizing TN mode in that both methods utilize the dielectric anisotropy, and accordingly because of their long switching time, the display devices utilizing this STN mode do not have sufficient characteristics as required for large screen display devices or devices for moving picture such as small-sized digital television.
In contrast thereto, R. B. Meyer et al. suggested in 1975 that organic compounds synthesized by them exhibit ferroelectric properties, and further suggested in 1980 that element comprising cells having a small gap filled with these ferroelectric liquid crystal compounds may be used as optical switching elements, i.e., display devices.
In distinction to switching elements utilizing TN mode or STN mode, the switching elements using such ferroelectric liquid crystal compounds as mentioned above are able to function as switching elements only by changing the direction of molecular orientation of said liquid crystal compounds and hence the switching time required for operating the switching elements is markedly shortened. Further, because a value of Ps.times.E obtained from a spontaneous polarization (Ps) of the ferroelectric liquid crystal compound and an intensity of the electric field (E) applied is an effective energy output for changing the direction of molecular orientation of said liquid crystal compound, power consumption required therefor can be markedly minimized. Such ferroelectric liquid crystal compounds as mentioned above are suitable particularly as display devices for large screen or moving picture, because they have two steady states depending upon the direction of electric field applied, that is, bistability and also very favorable switching threshold value characteristics.
When these ferroelectric liquid crystal compounds are intended for use in optical switching elements, they are required to have such characteristics as an operating temperature in the vicinity of ambient temperature (room temperature), a wide operating temperature zone, a high switching speed and an appropriate switching threshold value voltage. Particularly, of these characteristics, the operating temperature range is especially important when the ferroelectric liquid crystal compounds are used in optical switching elements
However, there are many ferroelectric liquid crystal compounds which are unusable for optical switching elements, because their operating temperature range does not agree with their service temperature range, in spite of the fact that other characteristics of said compounds are excellent [refer for example, to R. B. Meyer et al. J. de Phys., Vol. 36L, p. 69 (1975) and a paper reported by M. Taguchi and T. Harada, "Proceedings of Eleventh Conference on Liquid Crystal"p. 168 (1985)].